ON THE NATURE OF SOFT-X-RAY RADIATION IN COMETS

EUVE and ROSAT observations of Comet Hyakutake revealed emission of 7 x 10(24) photons per second in the soft Xray, We discuss and develop m ethods to calculate production of soft X-ray photons in cometary dust and gas by the following processes: (1) scattering and (2) fluorescenc e of solar X-rays; (3) K- and L-shell ionization by solar-wind protons and (4) electrons, and (5) by high-energy cometary ions; (6) bremsstr ahlung of solar-wind electrons; (7) cometary magnetospheric substorms; (8) collisions between cometary and interplanetary dust particles; (9 ) scattering, fluorescence, and bremsstrahlung by very small particles with mass on the order of 10(-19) g; and (10) charge transfer of the solar-wind heavy ions with cometary molecules suggested by T. E. Crave ns (1997, Geophys. Res. Lett. 24, 105-108). Very small attogram partic les were detected in Comet Halley by N. G. Utterback and J. Kissel (19 90, Astron. J. 100, 1315-1322) using the PUMA and PIA dust analyzers f rom the Vega and Giotto spacecraft. Of all these processes, only scatt ering by very small particles and charge transfer of the solar-wind he avy ions are capable of producing the measured soft X-ray emission. Ln the case of very small particles, the mean particle mass of 4.6 x 10( -19) g suggested by Utterback and Kissel implies that the total produc tion of these particles, which exceeds that of gas by a factor 3.6 (in mass), is inconsistent with the polarization and color of Comet Halle y, Both polarization and color require a reduction of the mean mass to (1-2) x 10(-19) g. This reduction strongly affects the visible bright ness of the par-tides, which is proportional to m(2), with a relativel y weak effect on the soft X-ray emission. We calculate the charge tran sfer process using the solar-wind ion densities and velocities in come t from both the model of T. I. Gombosi et al. (1994, J. Geophys. Res. 99, 21,525-21,539) and the Giotto measurements of Lu-particles and. He i ions in Comet Halley. The calculated emission constitutes 20-35% of the measured value. X-ray spectroscopy of the observed emission, spect roscopy of the He+ 304 Angstrom and He 522 Angstrom lines, and UV spec troscopy of cometary dust down to 1800 Angstrom may help in determinin g the contributions of the above processes. (C) 1997 Academic Press.